The neutralizing antibody response is an important factor that influences HIV-1 sequence diversity. Over time, variants that escape neutralization emerge, followed by the development of new neutralizing antibody responses to the variants. Data from out laboratory indicates that children with rapid progression of disease (RP) with low or undetectable autologous neutralizing antibody potency harbor viruses that over time are more sensitive to neutralization with monoclonal antibodies. Conversely, children with non-progression of disease (NRP) with a significant neutralizing antibody response harbor viruses that over time are more resistant to monoclonal neutralizing antibodies. The hypothesis of this study is that in the presence of a strong neutralizing antibody response the virus envelope adopts a conformation that makes it refractory to neutralization. When autologous neutralizing antibodies decline or are absent, the virus can adopt a conformation that allows higher replication rates and fitness. Therefore, neutralizing antibodies modulate viral fitness in vivo. To test this hypothesis, virus pairs from 8 children (4 RP and 4 NRP) with increasing or decreasing sensitivity to neutralization by monoclonal antibodies were selected. Recombinant viruses containing the envelope of the test viruses and the backbone of HXB2 will be prepared, and used to infect peripheral blood mononuclear cells singly of in competition with a clade A virus. The relative replication rates (fitness) of recombinant viruses will be measured using TaqMan real time PCR. Fitness of viruses with increasing or decreasing sensitivity to neutralizing monoclonal antibodies will be compared. To mimic the conditions found in vivo, virus isolates sensitive to monoclonal neutralizing antibodies will be propagated in vitro in the presence of these antibodies to select for neutralization resistant variants. Fitness of variants and parental isolates will be compared to test for a decrease in fitness of the resistant variants. Results of this study could provide for immunotherapies to reduce viral fitness in vivo.